In a number of airborne particle-measurement and particle-concentration studies, a condensation particle counter (CPC, also known as a condensation nucleus counter (CNC)) is used to detect particles in a monitored environment. In a CPC, particles can be detected that are too small to scatter enough light to be detected by conventional detection techniques (e.g., light scattering of a laser beam in an optical particle counter, OPC). The small particles are grown to a larger size by condensation formed on the particle. That is, each particle serves as a nucleation point for the working fluid; a vapor, which is produced by the instrument's working fluid, is condensed onto the particles to make the particles larger. After achieving growth of the particle due to condensation of the working fluid vapor onto the particle, CPCs function similarly to optical particle counters in that the individual droplets then pass through the focal point (or line) of a laser beam, producing a flash of light in the form of scattered light. Each light flash is counted as one particle.
However, in certain environments, such as air pollution measurements, engine exhaust research, and regulatory studies involving measurement of size or concentrations of particles in an aerosol stream, the concentration of particles is too high to measure accurately with a CPC. Often, such particle-measurement methods and procedures are defined by a governmental agency, such as the United States Environmental Protection Agency (EPA) or the California Air Resources Board (CARB). Often, a concentration of particles is too high to measure accurately with a CPC. In these environments, particle concentrations can range up to 5×109 particles per cubic centimeter or higher. However, many CPCs can only measure particle concentrations accurately at much lower concentration ranges (e.g., perhaps a factor of 104 lower than the stated particle concentration above) before inaccurate monitoring occurs due to coincidence errors (counting two or more particles in an aerosol sample stream simultaneously). Although various capillary-type and orifice dilutors are known in the art, none are capable of accurately providing a known dilution ratio under circumstances of varying temperature and absolute pressure. The problem is often compounded as the actual dilution ratio can vary as a function of time.